In addition to the silanes of the general formula Me.sub.a SiCl.sub.4-a, in which a has a value from 0 to 4 and Me here and below denotes a methyl group, small amounts of ethylchlorosilanes, various hydridosilanes, above all Me.sub.b HSiCl.sub.3-b, in which b has a value from 0 to 2, and ethyldichlorosilane EtHSiCl.sub.2 are also formed during direct synthesis of methylchlorosilanes from silicon and methyl chloride at 250.degree. to 300.degree. C. using copper catalysts. The direct synthesis is described, inter alia, in W. Noll, Chemistry and Technology of Silicones, Academic Press, Inc., Orlando, Fla., 1968, chapter 2.2.
The most sought-after target product of the direct synthesis is Me.sub.2 SiCl.sub.2, which can be converted by hydrolysis and polycondensation into silicone polymers having diverse functional groups and structures.
An essential feature of most silicone polymers is the lowest possible content of trifunctional impurities in the polymer skeleton. One of the possible trifunctional impurities of the Me.sub.2 SiCl.sub.2 employed is EtHSiCl.sub.2.
Since the boiling points of Me.sub.2 SiCl.sub.2 (70.degree.-71.degree. C.) and EtHSiCl.sub.2 (74.degree.-76.degree. C.) differ from one another by only about 4.degree. C., a very distillative effort, such as high reflux ratios, a large number of theoretical plates, and trays in practice, a high energy requirement and a reduced space/time yield, are necessary in order to obtain the Me.sub.2 SiCl.sub.2 in the purity required for the particular use.
EP-A 423,948 describes the removal of silanes containing hydrogen atoms bonded directly to silicon (H-silanes) from organosilane mixtures by reaction with hydrogen chloride gas in the presence of suitable catalysts from sub-group VIII of the Periodic Table, Pd, Pt, Rh, Ru, Ni, Os, Ir and compounds thereof, to give the corresponding alkylchlorosilanes. The difference in boiling points between the organosilanes desired and the impurity is increased by this measure such that the distillation can be operated with a considerably reduced effort.
The disadvantage of this known process is that the metals employed as catalysts, with the exception of nickel, are relatively expensive and that, if nickel is employed as the catalyst, relatively high catalyst concentrations are necessary and the conversions thereby achieved are low, and that the metals are in some cases too susceptible to catalyst poisons.